Site Area

Make sure that the Site Area includes all the pages in your website:

Could be something like this:

And exclude pages when the query string contains: preview_theme_id=65568381. This is very important, so when you activate the experiment, it does not cause a loop. If you see a loop, you will know what you missed.

Hide Shopify Preview Bar

Important

When the preview theme template is enabled there is a banner that appears specifying that it is a preview template.

However, this can be removed with CSS added to Shopify. In the section Online Store / Preferences / Google Analytics, add the following code: